Installer update

Moved the upgrade check, it was after the creation of the folder it
tests for.
Updated directory/folder creation in all installers, to reduce the
number of commands.
Updated mechanism for IP Address detection to use 'ip' command,
ifconfig is deprecated.
Added code to check if required software is already installed, to allow
installation on an unsubscribed RHEL system.
Updated rpm install commands to use 'yum', so yum doesn't complain
about software installed outside of yum.
Removed automatic shutdown, when creating an image, to allow for any
additional post-tasks to be performed.
Added additional cleanup commands, prior to creating installer (remove
.DS_Store, xattr) as they are not needed on linux systems.
Used includes for httpd on RHEL-based systems.
Used includes for SMB shares on new installs.
Added install commands for avahi, vim-common as they are required for
other things to work.
Added code to update the Netatalk configuration on RHEL-based systems.
The file copied in place had no effect.
Cleaned up user creation.
Added code to update smb printing in the smb.conf to prevent samba
logging excessively.
Updated dialog.sh to detect OS, rather than editing during the install
process.
Updated to append to rc.local on RHEL-based installs, rather than
overwriting it.
Enhancements for NetBoot

Subtle enhancements, broadcast image name, pre-populate subnet, ip /
subnet validation, restart / shutdown.